[Haskell] monadic where
John Meacham
john at repetae.net
Tue Sep 20 14:55:33 EDT 2005
On Tue, Sep 20, 2005 at 10:29:14AM +0300, Yitzchak Gale wrote:
> John Meacham wrote:
> > f x y
> > | b > c = ...
> > | c <= 0 && a > b = ...
> > where
> > a = ...
> > b <- ...
> > c <- ...
> > f x y = ...
> >
> > there is really no clean (IMHO) way to express this idiom otherwise,
>
> What does this translate to (even if not clean)?
> I am not sure I understand what this is supposed
> to do.
f x y = do
let a = ...
b <- ...
c <- ...
case b > c of
True -> ...
False -> case c <= 0 && a > b of
True -> ...
False -> (next fn)
>
> It looks like the guards depend on things computed
> inside the monad, so I guess you want the guard
> code put at the end of the monadic code.
yeah. exactly.
>
> But then, by the time you get to the guards, you may
> have already created side effects in the monad. So
> running the second version of f after failing the
> first version is not equivalent to running the second
> version by itself. Or am I missing something?
this is true. all the actions in a where clause will be executed before
pattern matching starts. to limit confusion, we could enforce only a
single choice, with pattern guards it is not even that onerous. but I
don't know if that is necessary.
John
--
John Meacham - ⑆repetae.net⑆john⑈
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